Cryogenic refrigerator and displacer

ABSTRACT

A cryogenic refrigerator includes a displacer including a body part and a heat conducting part, wherein the material of the heat conducting part has a higher thermal conductivity than the body part; a cylinder accommodating the displacer in such a manner as to allow the displacer to reciprocate in the axial directions of the cylinder, wherein an expansion space is formed between the cylinder and a low temperature end of the displacer; a clearance channel formed between the displacer and the cylinder so as to allow a refrigerant gas to flow into the expansion space; and a cooling stage positioned adjacent to the expansion space. The heat conducting part faces the cooling stage across the clearance channel.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of U.S. patent application Ser. No. 13/709,275, filed on Dec. 10, 2012, which is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2012-001627, filed on Jan. 6, 2012, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a cryogenic refrigerator that produces cryogenic temperatures by causing the Simon expansion using a high-pressure refrigerant gas fed from a compressor, and to a displacer used in the cryogenic refrigerator.

2. Description of the Related Art

For example, the cryogenic refrigerator described in Japanese Laid-Open Patent Application No. 2011-17457 produces cold temperatures by causing a refrigerant gas in an expansion space to expand with the opening and closing of a valve while causing a displacer to reciprocate inside a cylinder. The refrigerant gas is fed into and discharged from the expansion space through a clearance between the displacer and the cylinder. The refrigerant gas exchanges heat with a cooling stage positioned on the peripheral side of the clearance expansion space, so that the cold temperatures produced by the refrigerant gas in the expansion space cool an object of cooling connected to the cooling stage.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a cryogenic refrigerator includes a displacer including a body part and a heat conducting part, wherein a material of the heat conducting part has a higher thermal conductivity than the body part; a cylinder accommodating the displacer in such a manner as to allow the displacer to reciprocate in axial directions of the cylinder, wherein an expansion space is formed between the cylinder and a low temperature end of the displacer; a clearance channel formed between the displacer and the cylinder so as to allow a refrigerant gas to flow into the expansion space; and a cooling stage positioned adjacent to the expansion space, wherein the heat conducting part faces the cooling stage across the clearance channel.

According to an aspect of the present invention, a displacer includes a body part; and a heat conducting part, wherein the heat conducting part is positioned at a low temperature end of the displacer, and a material of the heat conducting part has a higher thermal conductivity than the body part.

The object and advantages of the embodiments will be realized and attained by means of the elements and combinations particularly pointed out in the claims,

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and not restrictive of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating a cryogenic refrigerator and a displacer according to a first embodiment of the present invention;

FIG. 2 is a schematic diagram illustrating a cryogenic refrigerator and a displacer according to a second embodiment of the present invention;

FIG. 3 is a diagram illustrating a variation of a heat conducting part of the cryogenic refrigerator and the displacer according to the second embodiment;

FIG. 4 is a schematic diagram illustrating another variation of the heat conducting part of the cryogenic refrigerator and the displacer according to the second embodiment; and

FIG. 5 is a schematic diagram illustrating a

two-stage cryogenic refrigerator according to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described above, Japanese Laid-open Patent Application No. 2011-17457 describes a cryogenic refrigerator that produces cold temperatures toy causing a refrigerant, gas in an expansion space to expand with the opening and closing of a valve while causing a displacer to reciprocate inside a cylinder. However, according to the technique described in Japanese Laid-Open Patent Application No. 2011-17457, the refrigerant gas that passes through the clearance exchanges heat only with the cooling stage positioned on the peripheral side of the clearance, thus making it difficult to ensure a sufficient substantial area for heat exchange.

According to an aspect of the present invention, a cryogenic refrigerator and a displacer are provided that make it possible to more effectively ensure a sufficient substantial area for heat exchange.

In a cryogenic refrigerator and a displacer according to an aspect of the present invention, heat enters a heat conducting part from, a cooling stage through a clearance channel on the periphery of the displacer. The heat conducting part transfers heat to a refrigerant gas that enters the clearance channel because of expansion. As a result, the temperature difference of the cooling stage is reduced, and a substantial heat exchange area that contributes to heat exchange increases, so that it is possible to improve heat exchange efficiency.

A description is given below, with reference to the accompanying drawings, of embodiments of the present invention.

[a] First Embodiment

A cryogenic refrigerator 1 according to a first embodiment is, tor example, a Gifford-McMahon (GM) refrigerator that uses helium, gas as a refrigerant gas. The cryogenic refrigerator 1 includes a displacer 2, a cylinder 4, and a cooling stage 5 that has a bottomed cylinder (tube) shape. A clearance C (a clearance channel) and an expansion space 3 are formed between the displacer 2 and the cylinder 4. The cooling stage 5 is adjacent to and encloses the expansion space 3. The displacer 2 includes a body part 2 a and a heat conducting part 2 b. The heat conducting part 2 b is formed of a material that has a higher thermal conductivity than the body part 2 a. The heat conducting part 2 b faces the cooling stage 5 across the clearance C. The cooling stage 5 is formed of, for example, copper, aluminum, stainless steel or the like.

Here, the heat conducting part 2 b has a lower coefficient of thermal expansion than the body part 2 a. The heat conducting part 2 b includes an overlapping part 2 ba that overlaps the body part 2 a in the directions of strokes (stroke directions) of the displacer 2 (in which the displacer 2 reciprocates). The body part 2 a includes an overlapped part 2 ab that corresponds to the overlapping part 2 ba. In the first embodiment, the heat conducting part 2 b has a two-stage (stepped) columnar shape, and the overlapping part 2 ba is formed by the second (upper) columnar shape from the bottom in FIG. 1.

The overlapping part 2 ba includes first hole parts 2 bah, and the overlapped part 2 ab includes second hole parts 2 abh that correspond to the first hole parts 2 bah. The body part 2 a and the heat conducting part 2 b are connected by press-fit pins 6 (an insertion member)

that are press-fit and inserted into both the second hole parts 2 abh and the first hole parts 2 bah. The press-fit pins 6 are provided at suitable points in a circumferential direction of the displacer 2. A material that has a higher thermal conductivity than at least the body part 2 a, such as copper, aluminum, stainless steel or the like, is used for the heat conducting part 2 b. The press-fit pins 6 may be either Bakelite (phenol containing cloth) or stainless steel. The overlapping part 2 ba is fixed to the overlapped part 2 ab by press-fitting the press-fit pins 6 into the first hole parts 2 bah and the second hole parts 2 abh.

The cylinder 4 accommodates the displacer 2 in such a manner as to allow the displacer 2 to reciprocate in the longitudinal directions of the cylinder A. For example, stainless steel is used for the cylinder 4 in terms of strength, thermal conductivity, helium blocking capability, etc.

A Scotch yoke mechanism (not graphically illustrated) that causes the displacer 2 to reciprocate is provided at a high-temperature end of the displacer 2, so that the displacer 2 reciprocates along the axial directions of the cylinder 4.

The displacer 2 has a cylindrical peripheral (exterior circumferential) surface. The displacer 2 is filled with a regenerator material. The internal space of the displacer 2 forms a regenerator 7. An upper flow rectifier 9 that rectifies (regulates) a flow of helium gas is provided on the upper end side, that is, the room temperature chamber 8 side, of the regenerator 7. A lower flow rectifier 10 is provided on the lower end side of the regenerator 7.

An opening 11 through which a refrigerant gas flows from a room temperature chamber 8 into the displacer 2 is formed at the high temperature end of the displacer 2, The room temperature chamber 8, which is a space defined by the cylinder 4 and the high temperature end of the displacer 2, changes in volume with the reciprocation of the displacer 2.

Of the various pipes that interconnect a compressor 12, a supply valve 13, and a return valve 14, which form a intake and outlet system, a pipe common to supply and discharge (a supply and discharge common pipe) is connected to the room temperature chamber 8. Further, a seal 15 is attached between part of the displacer 2 on the high temperature end side and the cylinder 4.

Openings 16 that introduce a refrigerant gas into the expansion space 3 via the clearance C are formed at a low temperature end of the displacer 2. The expansion space 3, which is a space defined by the cylinder 4 and the displacer 2, changes in volume with the reciprocation of the displacer 2, The cooling stage 5, which is thermally coupled to an object of cooling, is placed at a position corresponding to the expansion space 3 on the periphery and the bottom of the cylinder 4. The cooling stage 5 is cooled by a refrigerant gas that passes through the clearance C.

For example, Bakelite (phenol containing cloth) is used for the body part 2 a of the displacer 2 in view of specific gravity, strength, thermal conductivity, etc. The regenerator material is formed of, for example, a wire mesh. FIG. 1 illustrates the cryogenic refrigerator 1 in operation. Therefore, because of slight contraction of the body part 2 a due to low temperature, the body part 2 a and the heat conducting part 2 b are equal in outside diameter. However, at normal temperature (5° C. to 35° C.), the outside diameter of the heat conducting part 2 b is slightly smaller than the outside diameter of the body part 2 a.

Next, a description is given of an operation of the cryogenic refrigerator 1. At some point of time daring a refrigerant gas supply process, the displacer 2 is positioned at its bottom dead center inside the cylinder 4, When the supply valve 13 is opened at the same time with or slightly before or after that point of time, high-pressure helium gas is supplied into the cylinder 4 through the supply valve 13 and the supply and discharge common pipe, and flows into the regenerator 7 inside the displacer 2 through the opening 11 positioned at the top (high temperature end) of the displacer 2. The high-pressure helium gas that has flown into the regenerator 7 is supplied into the expansion space 3 through the openings 16, positioned in a lower part of the displacer 2, and the clearance C while being cooled by the regenerator material.

Thus, the expansion space 3 is filled with the high-pressure helium gas, and the supply valve 13 is closed. At this point, the displacer 2 is positioned at its top dead center inside the cylinder 4. When the return valve 14 is opened at the same time with or slightly before or after that point, the pressure of the helium (refrigerant) gas inside the expansion space 3 is reduced, so that the helium gas expands. The helium gas inside the expansion space 3, whose temperature has been lowered because of its expansion, absorbs the heat of the cooling stage 5 through the clearance C.

The displacer 2 moves toward the bottom dead center, so that the volume of the expansion space 3 is reduced. The helium gas inside the expansion space 3 is returned to the intake side of the compressor 12 through the clearance C, the openings 16, the regenerator 7, and the opening 11. During this process, the regenerator material is cooled by the refrigerant gas (helium gas). Letting this process be one cycle, the cryogenic refrigerator 1 cools the cooling stage 5 by repeating this cooling cycle.

According to the cryogenic refrigerator 1 and the displacer 2 of the first embodiment, the heat conducting part 2 b constantly faces the cooling stage 5 across the clearance C. Heat that enters from the cooling stage 5 also enters the heat conducting part 2 b via the helium gas in the clearance c. Therefore, when the low-temperature helium gas generated in the expansion space 3 passes through the clearance C, heat exchange is performed not only between the helium gas and the cooling stage 5 but also between the helium gas and the heat conducting part 2 b. As a result, it is possible to increase a substantial area for heat exchange (heat exchange area) between the cooling stage 5 and the low-temperature helium gas.

Further, the heat that has entered the heat conducting part 2 b is further transferred through the inside of the heat conducting part 2 b toward the expansion space 3. Therefore, it is possible to further improve heat exchange efficiency by configuring the heat conducting part 2 b so that the heat conducting part 2 b comes into contact with the low-temperature helium gas inside the expansion space 3.

In contrast, according to a configuration without the heat conducting part 2 b, that is, according to the conventional displacer where a part corresponding to the heat conducting part 2 b is formed of Bakelite, the heat exchange between helium gas and Bakelite is so limited that no substantial heat exchange is performed. Therefore, when low-temperature helium gas generated in an expansion space passes through a clearance, heat exchange is performed only between the helium gas and a cooling stage.

Thus, according to the cryogenic refrigerator 1 and the displacer 2 of the first embodiment, it is possible to cause the heat conducting part 2 b as well to effectively contribute to heat exchange, compared with the conventional displacer, so that it is possible to increase a substantial heat exchange area. Further, the above-described flow (transfer) of heat generated inside the heat conducting part 2 b makes it possible to further improve heat exchange efficiency. That is, it is possible to reduce the temperature difference of the cooling stage 5 in a vertical direction in FIG. 1. In particular, it is possible to reduce the temperature difference in the case of providing an object of cooling below the cooling stage

Further, when Bakelite is used for a part corresponding to the heat conducting part 2 b as in the conventional displacer, the part contracts with a decrease in temperature because of a relatively high coefficient of thermal expansion of Bakelite, so that a part corresponding to the overlapping part 2 ba may come off a part corresponding to the overlapped part 2 ab. In contrast, according the cryogenic refrigerator 1 and the displacer 2 of the first embodiment, the overlapping part 2 ba, which has a lower coefficient of thermal expansion than the body part 2 a, is provided inside (on the inner circumference side of) the overlapped part 2 ab of the body part 2 a. Therefore, when the overlapped part 2 ab of the body part 2 a is cooled to contract, a squeezing force is exerted on the overlapping part 2 ba of the heat conducting part 2 b, so that, it is possible to prevent the overlapping part 2 ba from coming off the overlapped part 2 ab.

Further, according to the cryogenic refrigerator 1 and the displacer 2 of the first embodiment, the heat conducting part 2 b also contributes to heat exchange, thereby increasing a substantial heat exchange area. Therefore, even when the cooling stage 5 and the clearance C are reduced in length in the axial directions (the moving directions of the displacer 2) compared with the conventional displacer, it is possible to obtain a desired refrigerating capacity. As a result, it is possible to reduce channel resistance and pressure loss in the clearance C, so that, it is possible to increase the refrigeration efficiency of the cryogenic refrigerator 1, Further, reduction in the volume of the clearance C leads to a decrease in dead volume that does not contribute to generation of cold temperatures. This may be expected to reduce a pressure difference between a high pressure and a low pressure during one cycle due to dead volume.

The overlapping part 2 ba and the overlapped part 2 ab may form a screw part so as to be screwed to each other. For example, the overlapping part 2 ba and the overlapped part 2 ab may be screwed to each other with their respective threaded parts mating with each other. This allows the body part 2 a and the heat conducting part 2 b to be more easily attached to and detached from each other, in this case as well, when the overlapped part 2 ab of the body part 2 a is cooled to contract, a squeezing force is exerted on the overlapping part 2 ba of the heat-conducting part 2 b. Thus, it is possible to further 15 prevent the overlapping part 2 ba from coming off the overlapped part 2 ab.

[b] Second Embodiment

In the above-described first embodiment, the heat conducting part 2 b has a columnar shape, while the heat conducting part 2 b may have a tubular shape as described below, FIG. 2 is a schematic diagram illustrating a cryogenic refrigerator 21 and a displacer 22 according to a second embodiment. In FIG. 2, the same elements as those of the first embodiment of FIG. 1 are referred to by the same reference numerals, and a description is basically given of differences from the first embodiment.

According to the second embodiment, the displacer 22 includes a body part 22 a and a heat conducting part 22 b, The heat conducting part 22 b has a tubular shape, and the entire heat conducting part 22 b forms an overlapping part 22 ba that overlaps the body part 22 a in the stroke directions of the displacer 22. A portion of the body part 22 a that is positioned on the low temperature side of the openings 16 (that is, below the openings 16 in FIG. 2) has a smaller diameter than a portion of the body part 22 a that is positioned on the high temperature side of the openings 16 (that is, above the openings 16 in FIG. 2). This smaller-diameter portion of the body part 22 a forms an overlapped part 22 ab that corresponds to the overlapping part 22 ba.

The overlapping part 22 ba includes first hole parts 22 bah, and the overlapped part 22 ab includes second hole parts 22 abh that correspond to the first hole parts 22 bah. The body part 22 a and the heat conducting part 22 b are connected by press-fit pins 26 (an insertion member) that are press-fit and inserted into both the second hole parts 22 abh and the first hole parts 22 bah. Like in the first embodiment, a material that has a higher thermal conductivity than at least the body part 22 a, such as copper, aluminum, stainless steel or the like, is used for the heat conducting part 22 b. In this embodiment as well, the press-fit pins 26 may be either Bakelite (phenol containing cloth) or stainless steel. The overlapping part 22 ba is fixed to the overlapped part 22 ab by inserting the press-fit pins 26 into the first hole parts 22 bah and the second hole parts 22 abh.

According to the cryogenic refrigerator 21 and the displacer 22 of the second embodiment as well, it is possible to increase a heat exchange area by causing the heat conducting part 22 b to contribute to heat exchange as in the first embodiment. In addition, in the second embodiment, the heat conducting part 22 b is placed only on the peripheral (outer circumferential) side of the displacer 22, which contributes to heat exchange compared with the first embodiment. Therefore, it is possible to reduce the volume and mass of the heat conducting part 22 b compared with the volume and mass of the heat conducting part 2 b of the first embodiment, and thus to reduce the mass of the entire displacer 22, which is a movable part, compared with the mass of the displacer 2 of the first embodiment.

Further, the reciprocation of the heat conducting part 22 b, which is a conductor, under the presence of a magnetic field generates eddy current, which causes heat generation, that is, copper loss. According to the second embodiment, the volume of the heat conducting part 22 b is relatively small. Therefore, it is possible to control generation of copper loss accordingly.

Further, the heat conducting part 22 b may be formed of a standardized tubular material. Therefore, it is possible to reduce cost compared with the first embodiment.

As described above, the generation of copper loss is expected to be controlled by reducing the volume of a conductor, while the generation of copper loss may also be controlled by controlling generation of eddy current by designing a shape. For example, FIG. 3 illustrates a variation of the tubular heat conducting part 22 b illustrated in FIG. 2, where a slit S is formed to make the heat conducting part 22 b discontinuous in its circumferential directions. In FIG. 3, (a) is a plan view and (b) is a side view of the variation of the heat conducting part 22 b. According to this configuration, it is possible to prevent eddy current from continuously flowing in the circumferential directions in particular, so that it is possible to control generation of copper loss more effectively.

Further, FIG. 4 illustrates another variation of the heat conducting part 22 b, As illustrated in FIG. 4, the heat conducting part 22 b may have a bottomed tube shape. The heat conducting part 22 b of a bottomed tube shape causes heat that has entered the heat conducting part 22 b from the cooling stage 5 to be exchanged between a bottom part 22 bb of the heat conducting part 22 b and the expansion space 3. As a result, it is possible to increase cooling efficiency compared with the cryogenic refrigerator 21 of FIG. 2.

[c] Third Embodiment

In the above-described first and second embodiments, single-stage refrigerators are illustrated, while an embodiment of the present invention may also be applied to a two-stage refrigerator as described below. FIG. 5 is a schematic diagram illustrating a cryogenic refrigerator 31 and first and second displacers 32 and 36 according to a third embodiment.

Like the cryogenic refrigerators 1 and 21 of the first and second embodiments, the cryogenic refrigerator 31 according to the third embodiment is a Gifford-McMahon (GM) refrigerator using helium gas as a refrigerant gas. As illustrated in FIG. 5, the cryogenic refrigerator 31 includes the first displacer 32 and the second displacer 36. The second displacer 36 is connected to the first displacer 32 in a longitudinal direction of the second displacer 36. As illustrated in FIG. 5, the first displacer 32 and the second displacer 36 are connected via, for example, a pin 33, a connector 34, and a pin 35.

A first cylinder 37 and a second cylinder 38 are formed as a unit. A low temperature end of the first cylinder 37 and a high temperature end of the second cylinder 38 are connected at the bottom of the first cylinder 37. The second cylinder 38 is coaxial with the first cylinder 37, and is a cylindrical member that has a smaller diameter than the first cylinder 37. The first cylinder 37 accommodates the first displacer 32 in such a manner as to allow the first displacer 32 to reciprocate in the longitudinal, directions of the first cylinder 37, The second cylinder 38 accommodates the second displacer 36 in such a manner as to allow the second displacer 36 to reciprocate in the longitudinal directions of the second cylinder 38.

For example, stainless steel is used for the first cylinder 37 and the second cylinder 38 in consideration of strength, thermal conductivity, helium blocking capability, etc. The second displacer 36 has a coating of an abrasion resistant resin such as fluororesin on the peripheral (exterior circumferential) surface of its metallic cylinder of, for example, stainless steel.

A Scotch yoke mechanism (not graphically illustrated) that causes the first displacer 32 and the second displacer 36 to reciprocate is provided at a high-temperature end of the first cylinder 37, so that the first displacer 32 and the second displacer 36 reciprocate along the first cylinder 37 and the second cylinder 38, respectively.

The first displacer 32 has a cylindrical peripheral (exterior circumferential) surface. The first displacer 32 is filled with a first regenerator material. The internal space of the first displacer 32 serves as a first regenerator 39. A flow rectifier 40 and a flow rectifier 41 are provided on and under the first regenerator 39. A first opening 42 through which a refrigerant gas flows from a room temperature chamber 69 into the first displacer 32 is formed at a high temperature end of the first displacer 32. The room temperature chamber 69, which is a space defined by the first cylinder 37 and the high temperature end of the first displacer 32, changes in volume with the reciprocation of the first displacer 32. Of the pipes that interconnect a compressor 43, a supply valve 44, and a return valve 45, which form an intake and outlet system, a pipe common to supply and discharge (a supply and discharge common pipe) is connected to the room temperature chamber 69. Further, a seal 46 is attached between part of the first displacer 32 on the high temperature end side and the first cylinder 37.

Second openings 43 that introduce a refrigerant gas into a first expansion space 47 via a first clearance C1 (a clearance channel) are formed at a low temperature end of the first displacer 32. The first expansion space 47, which is a space defined by the first cylinder 37 and the first displacer 32, changes in volume with the reciprocation of the first displacer 32. A first cooling stage 49, which is thermally coupled to an object of cooling (not graphically illustrated), is placed at a position corresponding to the first expansion space 47 on the periphery of the first cylinder 37. The first cooling stage 49 is cooled by a refrigerant gas that passes through the first clearance C1.

The second displacer 36 has a cylindrical peripheral (exterior circumferential) surface. The second displacer 36 is filled with a second regenerator material. The internal space of the second displacer 36 serves as a second regenerator 50, The first expansion space 47 and a high temperature end of the second displacer 36 are connected by a communicating passage (not graphically illustrated). A refrigerant gas flows from the first expansion space 47 into the second regenerator 50 through this communicating passage.

A third opening 52 that introduces a refrigerant gas into a second expansion space 51 via a second clearance C2 (a clearance channel) is formed at a low temperature end of the second displacer 36. The second expansion space 51, which is a space defined by the second cylinder 38 and the second displacer 36, changes in volume with the reciprocation of the second displacer 36. The second clearance C2 is defined by a low temperature end part of the second cylinder 38 and the second displacer 36. The second clearance C2 is larger than a clearance between the second displacer 36 having a helical groove 63 as described below and the second cylinder 38.

A second cooling stage 53, which is thermally coupled to an object of cooling (not graphically illustrated), is placed at a position corresponding to the second expansion space 51 on the periphery of the second cylinder 38. The second cooling stage 53 is cooled by a refrigerant gas that passes through the second clearance C2.

The first displacer 32 includes a body part 32 a and a heat conducting part 32 b. The heat conducting part 32 b is formed of a material that has a higher thermal conductivity than the body part 32 a. For example, 25 Bakelite (phenol containing cloth) is used for the body part 32 a of the first displacer 32 in view of specific gravity, strength, thermal conductivity, etc. h material that has a higher thermal conductivity than at least the body part 32 a, such as copper, aluminum, stainless steel or the like, is used for the heat conducting part 32 b.

The heat conducting part 32 b has a lower coefficient of thermal expansion than the body part 32 a. The heat conducting part 32 b includes an overlapping part 32 ba that overlaps the body part 32 a in the directions of strokes (stroke directions) of the first displacer 32. The body part 32 a includes an overlapped part 32 ab that corresponds to the overlapping part 32 ba.

The second displacer 36 includes a body part 36 a and a heat conducting part 36 b. The heat conducting part 36 b is formed of a material that has a higher thermal conductivity than the body part 36 a. For example, Bakelite (phenol containing cloth) is used for the body part 36 a of the second displacer 36 in view of specific gravity, strength, thermal conductivity, etc. A material that has a higher thermal conductivity than at least the body part 36 a, such as copper, aluminum, stainless steel or the like, is used for the heat conducting part 36 b.

The heat conducting part 36 b has a lower coefficient of thermal expansion than the body part 36 a. The heat conducting part 36 b includes an overlapping part 36 ba that overlaps the body part 36 a. in the directions of strokes (stroke directions) of the second displacer 36. The body part 36 a includes an overlapped part 36 ab that corresponds to the overlapping part 36 ba.

The first regenerator material is formed of, for example, a wire mesh. The second regenerator material is formed by holding a regenerator material such as lead spheres with felt and a wire mesh in the axial directions.

The helical groove 63 is formed on the peripheral (exterior circumferential) surface of the second displacer 36, The helical groove 36 has a starting end communicating with the second expansion space 51 through the second clearance C2, and helically extends toward the first expansion space 47.

According to the third embodiment as well, the first displacer 32 and the second displacer 36 include the heat conducting part 32 b and the heat conducting part 36 b at their respective cold (low) temperature ends. Both the heat conducting part 32 b and the heat conducting part 36 b have a two-stage (stepped) columnar shape. The heat conducting part 32 b is fixed to the body part 32 a with press-fit pins 54. The heat conducting part 36 b is fixed to the body part 36 a with press-fit pins 55. According to the third embodiment as well, for the reasons stated above with respect to the first and the second embodiment, it is possible to improve cooling efficiency by increasing a substantial heat exchange area for each of the first cooling stage 49 and the second cooling stage 53.

All examples and conditional language provided herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventors to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority or inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.

For example, in the above-described embodiments, a description is given of the case where the number of stages of a cryogenic refrigerator is one or two. However, the number of stages may be suitably selected, and may be, for example, three. Further, in the above-described embodiments, a description is given of the case where the cryogenic refrigerator is a CM refrigerator. However, embodiments of the present invention are not limited to the GM refrigerator, and may be applied to any refrigerators having a displacer, such as Stirling refrigerators and Solvay cycle refrigerators.

According to an aspect of the present invention, it is possible to improve the refrigeration, efficiency of a cryogenic refrigerator by improving its heat exchange efficiency by effectively increasing a heat exchange area that substantially contributes to heat exchange through a side clearance without increasing the length of a cooling stage in the axial directions of the cryogenic refrigerator. Accordingly, embodiments of the present invention may be applied to various kinds of cryogenic refrigerators. 

What is claimed is:
 1. A cryogenic refrigerator, comprising: a displacer having a cylindrical shape extending between a low temperature end and a high temperature end thereof, and including: a body part made of phenol containing cloth, a heat conducting part provided at a lower temperature side relative to the body part, said heat conducting part including a diameter smaller than an overlapped part of the body part so as to be inserted into an inner circumference of the overlapped part of the body part that overlaps the overlapping part in a stroke direction of the displacer, said heat conducting part being made of copper or aluminum, a regenerator material within an inner space thereof, and an opening provided at an outer peripheral surface of the displacer at the low temperature end so as to be connected to the inner space; a cylinder accommodating the displacer in such a manner as to allow the displacer to reciprocate in axial directions of the cylinder, wherein an expansion space is formed between the cylinder and a low temperature end of the displacer; a clearance channel formed between the outer peripheral surface of the displacer and an inner peripheral surface of the cylinder so as to allow a refrigerant gas to flow into the expansion space from the opening of the displacer; and a cooling stage positioned adjacent to the expansion space and having a hollow portion provided around an outer peripheral surface of the cylinder so as to enclose the expansion space and a bottom portion provided so as to face a bottom portion of the cylinder so as to enclose the expansion space, wherein the heat conducting part faces the hollow portion of the cooling stage across the clearance channel, wherein the cylinder and the high temperature end of the displacer define a room temperature chamber that changes its volume according to a reciprocating movement of the displacer in the axial directions of the cylinder, and wherein the cylinder and the displacer defines the expansion space that changes its volume according to the reciprocal movement of the displacer.
 2. The cryogenic refrigerator as claimed in claim 1, wherein the heat conducting part has a lower coefficient of thermal expansion than the body part.
 3. The cryogenic refrigerator as claimed in claim 1, wherein the entire part of the bottom portion of the heat conducting part directly faces the bottom portion of the cylinder.
 4. The cryogenic refrigerator as claimed in claim 1, wherein the heat conducting part and the body part are fixed to each other by inserting a pin into the overlapped part and the overlapping part.
 5. The cryogenic refrigerator as claimed in claim 1, wherein the heat conducting part has a lower coefficient of linear thermal expansion than the body part.
 6. The cryogenic refrigerator as claimed in claim 4, wherein the pin is made of the phenol containing cloth. 